The process involving emission of electrons on striking of light on the surface of a cathode or metal is known as the photoelectric effect.

The phenomenon of photoelectric effect was first observed by the famous German physicist – Heinrich Rudolf Hertz (1857 – 1894). He observed, in 1887, that when the surfaces of two electrically charged spheres were illuminated by the light, a spark would jump more readily between them. Thus, light somehow facilitated the escape of electrons.

During the years 1886 – 1900, the two German physicists – Wilhelm Hallwachs and Philips Lenard investigated the photoelectric effect in detail. They found that photocurrent varies with potential difference between the anode and cathode, and also with the intensity and frequency of the incident light. Hallwachs and Lenard found that no photoelectrons at all were emitted unless the incident light’s frequency was above a minimum value. This minimum value of the frequency of light incident upon a surface of cathode is defined as the threshold frequency.